Such systems are known per se. The patent EP 0 691 162 describes a nebulization system with a concentration nozzle in which a piezoelectric element immersed in water generates a mist of water droplets at the outlet of a nozzle that concentrates the ultrasound generated by said piezoelectric element at its outlet point; the mist is next taken away by a stream of air generated by a fan. This nozzle is as a general rule disposed vertically, with the focusing outlet pointing upwards; the nozzle may also be inclined, for example at 45°.